Twenty-three sportsmen were given one 45-minute exposure to flotation-REST and one exposure to chamber-REST on two occasions, incorporating random assignment to either flotation-REST followed by chamber-REST or vice versa. On each occasion, the Restricted Environmental Stimulation Technique (REST) procedure was followed immediately by testing experimentally induced pain to one arm using a blood pressure cuff. It was found that flotation-REST induced a significantly higher degree of altered states of consciousness (ASC), as measured with an instrument assessing experienced deviation from normal state (EDN), than did chamber-REST. Participants experiencing High EDN in the flotation-REST condition reported higher levels of both “experienced pain” and “experienced stress” than did those experiencing Low EDN. These results suggest that the particular distinguishing features of flotation-REST and chamber-REST may cause selective deviations from normal levels of consciousness, under experimental conditions, that may underlie the subjective experience of pain and stress thresholds.